Method to implement hysteresis in a MOSFET differential pair input stage

ABSTRACT

A simplified comparator circuit ( 10 ) having hysteresis and lower power requirements for its implementation. The circuit ( 10 ) includes 2 minimum-sized MOSFETs (MN 4 , MN 5 ) providing feedback from the circuit output to an input device (MN 1 ) body to produce hystereis, requiring very little power. This invention is suitable for applications not requiring a precisely set hysteresis magnitude.

FIELD OF THE INVENTION

The present invention is generally directed to MOSFET differential pair circuits, and more particularly to such circuits having hysteresis.

BACKGROUND OF THE INVENTION

Comparator circuits often are implemented with hysteresis from output to input to prevent noise on the input from causing multiple switches in output, or chatter. There have been many means devised to accomplish the hysteresis, such as using a resistor network from input to output dissipating power in at least one state. Examples of some are shown in U.S. Pat. No. 4,110,641 to Robert L. Payne entitled “CMOS Voltage Comparator with Internal Hysteresis” and U.S. Pat. No. 4,394,587 to McKenzie et al entitled “CMOS Differential Comparator with Hysteresis”. Such previous solutions have required the addition of several devices creating a more complex design and requiring additional power.

SUMMARY OF THE INVENTION

The present invention achieves technical advantages as a simplified comparator circuit having hysteresis and lower power requirements for its implementation. The circuit includes 2 minimum-sized MOSFETs providing feedback from the circuit output to an input device's body to produce hystereis, requiring very little power. This invention is particularly suitable for applications not requiring a precisely set hysteresis magnitude.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an improved simplified comparator circuit having hysteresis according to the present invention; and

FIG. 2 is a timing diagram of the circuit of FIG. 1 showing the hysteresis and low static power.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is the schematic of a new comparator 10 with hysteresis according to a preferred embodiment of the present invention. Transistors MN1 and MN2 form a input differential pair. Transistor MN3 provides a tail current, and transistors MP1 and MP2 provide a mirror of transistor MN2's drain current.

At balance, when voltages at inputs VINP and VINM are equal, currents in transistors MN1 and MP1 will be equal and the output at transistor MP0's drain will be at an intermediate voltage between VCC and GND. If input VINP is slightly higher than input VINM, then the balance will be disturbed and the output OUT will go high. Any noise at the inputs near this balance point will cause the output OUT to reflect that same noise gained up.

In order to remove this noise at the output OUT, it is desirable that once the output OUT goes high the first time, hysteresis modifies the input threshold such that the input differential voltage needs to drop considerably before reaching the new input threshold. This threshold change needs to be outside the range of expected input noise.

According to one embodiment of the present invention, the back gates of transistors MN1 and MN2 are nominally tied to GND. The back gate of transistor MN2 is tied directly to GND, while the back gate of transistor MN1 is tied to GND through intermediate transistor MN4 providing a pull-down of 10 μA. Transistor MN4 is selectively enabled by a control signal CTRL, which CTRL signal is a bias line to define the DC currents in MN0, MN3 and MN4. Control signal CTRL is at GND potential when the comparator is biased off, and it is about 1V when the comparator is baised on. This pull-down current responsively goes to zero when transistor MN2's back gate is pulled down to GND. This condition exists when the input differential voltage is negative (VINP<VINM), the output OUT is low and transistor MN5 is off, as shown in FIG. 2.

Once the input differential voltage exceeds zero (VINP>VINM), the output OUT goes high, transistor MN5 consequently turns on and pulls transistor MN1's back gate voltage to that of its source. This, in-turn, advantageously causes the threshold of transistor MN1 to reduce compared to if its back gate was tied directly to GND. This threshold shift is the amount of hysteresis seen at the input VINP/VINM, and notably is a function of the body effect of the NMOS devices MN1 and MN2 in a particular semiconductor manufacturing process, and the common-mode voltage at the inputs VINP/VINM at the threshold. In this embodiment of the present invention, there is about a 0.5 volt Hysteresis.

Advantageously, the simple MOSFET differential circuit 10 having hysteresis is accomplished with the addition of only two small NMOS devices MN4 and MN5, and the only extra current is 10 μA, which current is most of the time off in applications that this comparator 10 may be used in, such as a delay circuit involving the charging of a capacitor. Thus, the static power dissipation if very low.

Though the invention has been described with respect to a specific preferred embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present application. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications. 

1. A comparator circuit, comprising; a differential pair of MOSFET transistors comprising a first transistor and a second transistor receiving a differential input signal and producing an output signal having hysteresis at an output; wherein the first and second transistors each have a back gate, the output being coupled to a back gate of the first transistor.
 2. The comparator circuit as specified in claim 1, wherein the back gate of the first transistor is sourced through a gate to ground.
 3. The comparator circuit as specified in claim 2, wherein the back gate of the second transistor is coupled directly to ground.
 4. The comparator circuit as specified in claim 2, wherein the gate is a third transistor.
 5. The comparator circuit as specified in claim 4, wherein the third transistor is enabled by a control signal.
 6. The comparator circuit as specified in claim 4, wherein the fourth transistor pulls the first transistor back gate voltage to the voltage of first transistor source.
 7. The comparator circuit as specified in claim 6 further comprising a fourth transistor enabled by the output signal such that the first transistor back gate has approximately the same voltage as the first transistor source.
 8. The comparator circuit as specified in claim 2, wherein a threshold of the first transistor is reduced as compared to if the first transistor back gate was to be directly grounded.
 9. The comparator circuit as specified in claim 8, wherein the threshold reduction of the first transistor is seen at the input of the differential pair of transistors.
 10. The comparator circuit as specified in claim 9, wherein the first transistor threshold reduction is the amount of hysteresis seen at the input of the differential pair of transistors.
 11. The comparator circuit as specified in claim 10, wherein the hysteresis is a function of a body effect of the first and second transistors.
 12. The comparator circuit as specified in claim 11, wherein the hysteresis is a function of a common-mode voltage at the input of the first and second transistors at the threshold.
 13. The comparator circuit as specified in claim 6, wherein the third and fourth transistors are NMOS transistors. 